A number of mechanical seals for vessels are known including seals around rotating shafts that pierce the walls of the vessels. The primary purpose of such seals is to prevent the contents of the vessels from leaking out the entry point of the shaft. However, due to the wide variety of liquids commonly processed, seals suitable for one application are not necessarily suitable for other uses.
An area of particular concern are mechanical seals for vessels that contain relatively large portions of very viscous liquids, such as a viscous liquid polymers. Such materials tend to subject the seals and the shafts therein which may drive a mixing blade or the like, to hydraulic shock during processing. Seals for such vessels are known, but these seals are also known to have relatively limited life spans, of only a few months, before the process line must be shutdown to replace the seals for excessive leaking due to seal deterioration.
A number of patents are known describing various vessel seals, but not all are suitable to seal around a rotatable shaft such as for a mixing blade for vessels containing very viscous materials. Such patents include U.S. Pat. No. 4,010,960 which discusses a rotating seal, sealing by viscous effect, installed on a rotating shaft crossing through a bore separating, from the outside medium, an enclosure containing a fluid kept under pressure. The seal has at its center two threads having an unequal length and having reverse pitches, separated by a groove and a lip seal for each around a rotating shaft of the conventional type on either side of the threads. A gap between the lip seals is fed with a viscous fluid by two tanks, one of which is at the pressure of the fluid for which sealing is to be provided. This device is said to have application to shafts rotating at a high speed for all types of enclosures containing a gas or a liquid under pressure which can reach several tens of bars.
A visco-type seal wherein a sealing pressure is built up in a fluid by means of one or more helical windings, the fluid being enclosed in a narrow clearance between a stator and a rotor is described in U.S. Pat. No. 3,963,247. The alignment of the rotor within the stator is insured by providing a thin layer of material at the outer ends of at least one of the helical windings so that the clearance in a radial direction between the stator and the rotor at this location is less than the clearance existing in a radial direction between the helical windings and the stator. In one embodiment, the stator is designed as a sleeve which has a flexible connection in the radial direction with respect to a wall. The stator or a sealing element fastened thereto is movable in an axial direction by means of a resilient force to obtain a proper sealing when the device is at rest.
Also of interest is U.S. Pat. No. 3,700,247 which mentions certain shaft seals using a viscoseal-type sealing screw such as those adjacent the pressurized screw pump section of a viscous liquid devolatilizer. Excessive heat buildup and/or prolonged elevated temperature exposure in the seal region can either deteriorate the liquid materials in the sealing screw or render such liquids less viscous to an extent such that the seal is no longer fully operative. To cool and flush the sealing screw and the liquid materials therein, a fluid conduit bringing fresh liquid material continuously to the sealing screw from the adjacent pressurized section is provided.
A number of seals use a reverse thread to pump the liquid away from the seal, as mentioned. For example, a packing for a rotor shaft opening through the casing of a machine for treating plastic materials is described in U.S. Pat. No. 1,552,668. The packing comprises in combination with the rotor shaft, three metal rings surrounding the shaft and arranged face to face. The inner of the rings has an opening larger in diameter than the shaft and is fixed against rotary, transverse and longitudinal movements relative to the shaft. The outer of the rings has an opening larger in diameter than the shaft and is fixed against rotary and transverse movements, but is capable of longitudinal movements relative to the shaft. The intermediate of the rings is closely fitted to and keyed to the shaft, the ring being capable of rotation with and movements transversely with the shaft between the inner and outer rings. A mechanism for forcing the rings into close contact is also present as are spiral grooves along the shaft to bring material back into the chamber.
U.S. Pat. No. 3,468,548 mentions a liquid-vapor interface type of seal for the rotating shaft of a turbine utilizing mercury as the working fluid. The seal includes three pumps extending between the shaft and the housing from the high-pressure turbine cavity to a low-pressure vented area. The pumps are adapted to pump liquid or vapor upon rotation of the shaft. First pump comprises a helical groove extending from the cavity to a slinger pump housed in a slinger space. The first pump is adapted to pump mercury vapor from the cavity toward the slinger space. The slinger pump tends to pump in the opposite direction of the first pump. The mercury vapor in the first pump and the slinger space is cooled and condensed to a liquid by flowing coolant liquid through a passage in the housing or by injecting cooled mercury into the seal. A liquid-vapor interface is formed in the slinger space and leakage of mercury vapor toward the vent is further restricted by means of a molecular pump in the form of a second helical groove extending from the vent to the slinger space and is adapted on rotation of the shaft to pump mercury molecules toward the interface.
A partly melting rotating seal assembly surrounding a rotating shaft crossing through a wall limiting a volume of fluid kept under pressure, constituted by a solidified sleeve of the fluid to be sealed or of an intermediate fluid, contained in an external metallic sleeve integral with the wall and provided with a cooling mechanism is disclosed in U.S. Pat. No. 3,940,150. A very thin thread, integral with the shaft, maintains during the rotation of the shaft, a thin liquid film between the shaft and the solidified sleeve, ensuring sealing along the shaft.
A mechanical seal for sealing a slurry liquid between a casing and a rotary shaft is described in U.S. Pat. No. 4,545,588. The seal comprises a fixed seat ring; a rotatable slide ring in the casing and abutting against the seat ring to form a sealing face therebetween; a helical spring in the casing and surrounding the rotary shaft, and biassing the rotatable slide ring toward the seat ring; and a rotatable ring on the rotary shaft and rotatable with the rotary shaft and holding the spring at a predetermined position relative to the rotary shaft, the spring being interposed between the rotatable slide ring and the rotatable ring. A small space is provided between the casing and the rotatable ring and the rotatable ring has a helical projection on the outer surface thereof. The helical projection of the rotatable ring and the helical spring are directed such that upon rotation thereof a force is produced in a direction away from the sealing face and toward the interior of the casing. The spring is a helical screw-thread-like member of rectangular cross-section.
Somewhat less relevent patents include U.S. Pat. No. 3,697,088 which discusses a mechanical seal construction for use between a wall and a shaft extending therethrough and rotatable relative to the wall. The seal construction includes a conventional mechanical seal assembly having a pair of seal members with mutually contacting seal faces thereon, one seal member being fixed with respect to the wall and the other seal member being capable of rotation with the shaft. A floatable bushing assembly is disposed adjacent the mechanical seal assembly and includes an annular bushing member surrounding the shaft and floatable relative to a surrounding stationary housing member. The bushing member is resiliently urged against the housing member to sealingly engage the same. A small radial clearance is provided between the bushing member and the shaft to permit relative rotation therebetween whereas the bushing is free to radially float in response to deflection or whip of the shaft. The floating bushing assembly is disposed outwardly from the mechanical seal assembly to act as a safety or back-up for preventing large quantities of fluid from suddenly escaping in the event of a failure of the mechanical seal assembly.
An improvement in a sealing combination for sealing intermediate a cap ring on the hydraulic cylinder or the like and a rod running concentrically therethrough is described in U.S. Pat. No. 4,476,772. The combination has in addition to the usual cage assembly on the cylinder assembly, a stuffing box and a wiper ring box for holding respective seals and ring wiper, bearing for maintaining the rod centralized within the central assembly and facilitating longitudinal movement. Also present is a primary seal disposed about the rod and within the sealing box, a rod wiper disposed about the rod and within the wiper ring box, secondary seal disposed about the rod and intermediate the primary seal and the rod wiper and a stop device disposed intermediate the primary seal and the secondary seal and adapted to stop longitudinal movement of the bearing to prevent crushing the second seal.
Finally, U.S. Pat. No. 4,648,605 discusses a mechanical seal assembly especially adaptable for use in the chemical industry in which a jacket impervious to the chemical involved surrounds a spring means which urges the seal faces toward one another. The spring is capable of transmitting torque and provides axial support to the jacket.
Despite these devices, there remains a need for a mechanical seal for rotating shafts on vessels which contain viscous liquids such as polymers with increased durability. Conventionally available seals often last only about four months. Study of the aforementioned patents will reveal that many are of complex design having numerous elements within the seal contact areas which are more likely to contribute to diminished, rather than increased durability.